An intruder stumbling across the track was the most alarming obstacle presented to Sebastian Vettel on his way to a serene victory in the Singapore Grand Prix.

It certainly was not a Mercedes or Lewis Hamilton. The world champion retired with a car failure, allowing Vettel (left) and Nico Rosberg, Hamilton's team-mate, to cut his lead in the standings. The door is just about ajar, particularly if Mercedes' bewildering lack of speed persists.

First, though, to the 27-year-old man, who left his indelible mark on this race. The imposter, thought to be English, staggered on to the track on lap 37 at Esplanade Drive, where the cars accelerate towards 180 mph. According to Vettel, the man took a quick photograph before hopping over the barrier.

The man was arrested, while the FIA ordered an urgent investigation into how such a deeply embarrassing breach of security could have occurred.

Vettel said: "I think I saw him take a picture. I hope it was a good one at least, in focus. It was pretty crazy. Maybe we need to block the grandstand a bit better. Fortunately nothing happened to me or him."

The four-time champion was not alone in expressing concern after the second incident of its kind this year. A man ran across the pit straight in practice for the Chinese Grand Prix trying to jump into Ferrari's garage. It also brought back memories of Silverstone 12 years ago, when defrocked priest Neil Horan ran along the Hangar Straight.

Jenson Button, a director of the drivers' association, said: "You can make the barriers only a certain height but you've got to trust in the human race to be sensible."

The incident led to the safety car being brought out, which cost Daniel Ricciardo any chance of overhauling Vettel in the second round of pit stops. However, even this intruder could not stop Vettel, with the German leading from pole.

Meanwhile, the mystery as to how Mercedes could go from being the dominant car two weeks ago to some way off the pace here continues.

The best explanation, but by no means the most conspiratorial -remains how the Mercedes uses its rubber.

The Mercedes head of motorsport Toto Wolff admitted they had no explanation but expect it to be a blip. He said: "You cannot write it off as a one-off, and on the other side you can't say we are terrified now by this.